1. Field of the Invention
The present invention relates to an optical pickup actuator. More particularly, the present invention relates to an optical pickup actuator with two bobbins in which a tracking coil and a focusing coil are directly wound around the bobbin.
2. Description of the Prior Arts
Data recording/reproducing apparatuses using optical disks such as a laser disk (LD) and a compact disk (CD) have been commercially available in recent years. In order to read out data from an optical disk, a laser beam is irradiated onto a data recording track (to be referred to as a "track" hereinafter), and data are reproduced based on the beam reflected by the track.
When the tracks are helically formed on the optical disk, since the sectors of a single track are not equidistant from the center of the rotation of the disk, tracking (radial) control is necessary in the read mode to accurately irradiate the track with a laser beam.
This tracking control has been conventionally performed by one beam method or three beam method. Tracking errors are detected from the laser beam reflected by an optical disk. Tracking control operation is conformed by transferring the objective lens in response to the tracking error signals derived by these tracking error signals. The objective lens is normally supported and fixed by a spring on an optical head housing. A tracking actuator is energized to move the lens for the tracking control. When the tracking actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force.
Meanwhile, since the distance from the optical pickup to disk shifts minutely in the read mode in which the disk is rotated, it is difficult to correctly read data due to the shift, thus rendering focusing control essential. This focusing control has been conventionally performed by astigmatic method using astigmatism or a knife edge method.
For focusing control, focusing errors are detected from the laser beam reflected by an optical disk, and focusing control operation is conformed by transferring the objective lens in response to the focusing error signals derived by these focusing error signals. The objective lens is normally the same lens that is used for the tracking control. A focusing actuator is energized to move the lens for the focusing control. When the focusing actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force.
The conventional optical pickup actuators are generally classified into actuators of two types in arrangement of coils. The objective lens is moved in vertical (focusing) direction and horizontal (tracking) direction using optical pickup actuator. For example, there is one type that focusing and tracking coils are placed in the orthogonal and parallel directions, respectively. There is the other type that the two coils are placed in directions which are 45.degree./45.degree. against optical axis as in Noiman's cutter head, respectively.
These conventional optical pickup actuators for moving the objective lens in vertical and horizontal directions for focusing and tracking control as shown above are described in U.S. Pat. No. 5,103,438 (issued to Masunaga et al.), U.S. Pat. No. 5,182,738 (issued to Yoshigawa), etc.
As shown in FIGS. 1 and 2, two suspension wires 2 extend in parallel with each other in a horizontal direction. One end of each of the suspension wires 2 is connected to an upstanding wall portion of an actuator base 1. Further, a holder 4 is positioned above the actuator base 1 for supporting an objective lens 3, and the holder 4 is connected to each end of the suspension wires 2. Therefore, the holder 4 is movably supported on the suspension wires 2 in a cantilevered fashion.
Magnets 5A and 5B are fixed to the holder 4. Further, focusing coils 7A and 7B and tracking coils 8A and 8B are mounted on yokes 6A and 6B which stand vertically from the actuator base 1. One magnet 5A confronts the focusing coil 7A and tracking coil 8A, and the other magnet 5B confronts the focusing coil 7B and tracking coil 8B. The combination of holder 4, the coils 7, 8, the magnets 5, and suspension wires 2 is generally referred to as an actuator.
With such an actuator, the suspension wires 2 are formed of extremely flexible materials having low elastic modulus so as to insure precise movement of the holder 4 in response to the magnetic forces. Stated differently, in the conventional actuator, a sufficiently high magnetic flux density has not been obtainable, and therefore, the suspension wires 2 have required extremely high flexibility so that the wires 2 do not restrain movement of the holder 4 in response to the generated magnetic attractive force.
According to the optical pickup actuator having the wire type suspension, the laser beam generated from the laser diode is incident on the recording pit, and the laser reflected from the recording pit is received on the photo detector. As a consequence, information recorded on the recording pit is read out. The tracking and focusing conditions on the recording pit are detected by the laser beam reflected from the recording pit.
When focusing control is required, a current is supplied to the focusing coils 7A and 7B in clockwise and counterclockwise directions. At that time, electromagnetic force acts in a direction of the support shaft 6 (focusing direction X). Accordingly, the objective lens is axially moved to correspond with a change in surface level of the optical disc, so that the beam spot follows the recording surface of the disc.
When tracking control is required, a current is supplied to the tracking coils 8A and 8B in one direction or in the reverse direction thereof. At that time, electromagnetic force acts in an orthogonal direction with the support shaft (tracking direction Y). Accordingly, the beam spot traces the recording tracks in accordance with eccentricity of the tracks.
Thus, the predetermined currents flow into focusing and tracking coils, which are respectively move the holder 4 in the focusing direction X or the tracking direction Y. As a result, the focusing and tracking servos are carried out.
According to the optical pickup actuator, during the tracking servo, the lens holder connected with the suspension wires may be moved in the tracking direction. At this time, since the distances between lens holder and both tracking coils are different from each other, the forces applied to the lens holder by both tracking coils are minutely different from each other. Thereby, minute malfunction generates in the tracking servo.
Furthermore, since the lens holder of actuator is provided with a magnet fixed at side faces thereof, the suspension wires have to be formed from a material capable of bearing a heavy weight. Therefore, it requires the larger force during tracking or focusing controls, and it makes difficult minute controlling.
During the manufacture of the optical pickup actuator, the tracking coils are temporarily installed on the sides of the winding focusing coils. The function of the tracking coils is checked under the temporarily fixed state, and then the appropriate positions of the tracking coils are determined to be adhered and fixed at the positions.